1. Field of the Invention
The present invention relates to a sheet processing apparatus that performs sheet processing on a sheet conveyed onto a tray and to an image forming apparatus including the same and, in particular, to one having a buffering mechanism that temporarily buffers a sheet.
2. Description of the Related Art
In recent years, some image forming apparatuses, such as copiers, printers, and facsimile machines, can perform additional processing on a sheet with an image formed thereon by use of sheet processing apparatuses that are connected in a sheet conveying direction and that perform sheet processing, such as binding, punching, and sorting, on a sheet with an image formed thereon. To perform sheet processing requiring a long processing time, for example, binding processing, a sheet processing apparatus described above temporarily stores (hereinafter referred to as buffers) a succeeding sheet conveyed from the image forming apparatus during the processing of a preceding sheet bundle in order to avoid reduction in productivity. A processing time required for the sheet processing can be obtained by buffering a necessary number of succeeding sheets for obtaining the processing time for the preceding sheet bundle and causing a sheet processing portion to convey the buffered sheets in a piled state after the preceding sheet bundle is discharged. In such a way, while sheet processing is performed on a preceding sheet bundle, succeeding sheets are buffered by a buffering mechanism, and thus during that sheet processing, an image forming operation in an image forming apparatus is not stopped, thereby improving manufacturing capability in the entire system.
An example sheet processing apparatus having a buffering mechanism is the one in which succeeding sheets conveyed during sheet processing of a preceding sheet bundle on a processing tray are buffered in a buffer path and the succeeding sheets are discharged to the processing tray after the sheet processing (see Japanese Patent Laid-Open No. 11-322165). With this, even if sort processing is performed on a processing tray, for example, there is no need to stop an image forming operation in an image forming apparatus, so a processing time for executing a job containing sheet processing in the entire system is shortened.
There is also an apparatus that includes a sheet property detection portion for detecting a sheet property and prohibits a buffering operation of conveying a plurality of sheets in a piled state when the sheet property detection portion has detected that a conveyed sheet has a property different from that of a normal sheet (see Japanese Patent Laid-Open No. 2003-300658). This deals with a situation in which, in a traditional buffering operation, although it is difficult to pile sheets without misalignment and thus aligning the sheets again on a processing tray is necessary, depending on a sheet shape or a sheet property, such as a sheet surface nature, it may be difficult to collectively align a bundle in which sheets are piled.
For example, data on adhesion forces acting between sheets having different surface natures, like plain paper and (five types of) coated paper having coated surfaces, is illustrated in FIG. 20. These adhesion forces indicate forces required to, after sheets are conveyed in a piled state, displace the piled sheets in a sheet conveying direction to separate them. FIG. 20 reveals that adhesion forces of coated paper are significantly larger than that of plain paper. It is difficult to collectively align a bundle of sheets adhering to each other with such a large adhesion force on a processing tray. Therefore, for a traditional sheet processing apparatus, it is an object to achieve both stable conveyance and processing and improved efficiency of sheet processing of an entire system by determining whether a buffering operation is necessary by the property of a sheet conveyed to a processing tray and switching control in response to the determination.
However, for a sheet processing apparatus that determines the necessity or non-necessity of a buffering operation in accordance with the sheet property, running with reduced productivity of the entire system may be required depending on the property of a conveyed sheet, and this causes a decrease in performance. In particular, in recent years, there are situations where the variety of sheets that copiers and printers can use has increased, the number of sheets output per unit time has increased, and efforts to address environmental issues, for example, energy conservation, are advancing year by year. Under the circumstances, issues caused by being unable to perform buffering, such as a decrease in performance of an entire system and an increase in power consumption resulting from an increase in operation time, are becoming noticeable. Thus, irrespective of a sheet type, high throughput and high processing efficiency as much as possible are increasingly desired.
Accordingly, it is an object of the present invention to provide a sheet processing apparatus that reduces adhesion of sheets being conveyed in a piled state even when the sheets are coated paper having good surface nature or film sheets, such as overhead transparency (OHT) sheets, and that achieves satisfactory alignment of the sheets on a processing tray.